1. Field of the Invention
The invention relates generally to current steering circuits for variable gain amplifiers ("VGA's"). More particularly, the invention provides a circuit which improves the linearity (in the dB scale) of the output of a current steering circuit.
2. Description of the Prior Art
VGA's are used in numerous electronic products such as global positioning (GPS) receivers, wireless local area networks and mobile communication devices, such as cordless and cellular phones. In particular, VGA's are used in various parts of such devices, for example, in the radio frequency (RF), intermediate frequency (IF) and the low frequency, or baseband circuits, of these devices.
FIG. 1 illustrates a prior art variable gain amplifier current steering circuit 100 having a differential pair (bipolar transistors Q1, Q2) which steers a portion of an input current signal "I.sub.signal " received at input 102 to the output terminal 104, while the other, unused, portion of the input current signal is directed to ground. The amount of signal current that gets steered to the output depends on the differential control voltage "V.sub.agc " applied to the bases of the transistors Q1, Q2 via the inputs 101, 102. The portion of the input signal current that flows through the transistor Q2 appears at the output terminal, while the remainder flows through the transistor Q1 to ground. In general, applying a higher control voltage to the base of the transistor Q2, as compared to the base of the transistor Q1, will allow a larger portion of the input signal current to be available at the output 104.
The formula for the gain control characteristic of the circuit 100 results from the translinear equations of the circuit loop closed by the base-emitter junction of the transistor Q1, the base-emitter junction of the transistor Q2 and the control voltage V.sub.agc present between the input terminals 101 and 102. The resulting expression is ##EQU1## where I.sub.c1 and I.sub.c2 are the collector currents of transistors Q1 and Q2, respectively, and V.sub.T is the thermal voltage, 26 mV at room temperature. Since I.sub.c1 +I.sub.c2 =I.sub.signal (disregarding the base currents), the gain control expression which follows from Eq. 1 is ##EQU2## For gain control voltages V.sub.agc that are much smaller than 0, Eq. 3 shows a nice control characteristic in which the gain of the current steering circuit 100 is exponentially dependent on the control voltage. ##EQU3## This exponential relationship is commonly referred to as linear-in-dB, since plotted on a dB scale Eq. 3 yields a straight line in a region where V.sub.agc &lt;&lt;0. Unfortunately, going to higher voltages for V.sub.agc, the linear-in-dB relationship of Eq. 2 does not hold anymore. At control voltages of about 0 volts and higher, the gain control curve starts flattening out and approaches unity gain.
The current steering circuit of FIG. 1 and the above analysis (equations 1-3) are known from the text: B. Gilbert, Current-Mode Circuits from a Translinear Viewpoint: a Tutorial, at Chapter 2 (United Kingdom 1990).
The non-linearity of the control characteristic (on a dB-scale) as the control voltage approaches and exceeds zero volts is an important drawback of the circuit for use in variable gain amplifier (VGA) applications, because it decreases sensitivity of the signal gain to control voltage variations at high gain settings.
Accordingly, it is an object of the invention to improve the linearity (in the dB scale ) of the output of a variable gain amplifier.